The equine infectious anemia (EIA) is one of the oldest diseases caused by virus, having been described for the first time In France by LIGNEE, Rec. Med. Vet., 20:30, 1843, and recognized as viral disease by VALLEE and CARRE. Acad. Sci., 139:331-333,1904. The disease affects exclusively the members of the family Equidae presenting a worldwide distribution and of great economical importance consequently.
The EIA virus (EIAV) is classified as a lentivirus of the Retroviridae family (CHARMAN et al. J. Virol. 19(2):1073-1076,1976), it is genetic and antigenically related to the other lentiviruses that are characterized by developing persistent infection in host. The EIA has played an especially important role in comparative virology and in the studies of the acquired immunodeficiency syndrome (AIDS). Besides their morphological identity, both viruses are similar in terms of nucleotide sequences that code for structural surfaces' proteins. This group of viruses present genetic and antigenic variants during persistent infections, which are associated to the immunresponse scape. (MONTAGNIER et al. Ann. Virol., 135:119-134, 1984, MONTELARO et al. J. Biol. Chem., 259:10539-10544,1984, RUSHLOW et al. Virology, 155:309-321, 1986, STREICHER et al. J. Am. Med. Assoc. 256:2390-2391, 1986, STOLER et al. J. Am. Med. Assoc. 256:2360-2364,1986 and HAHN et al. Science, 232:1548-1553, 1986.
The transmission of EIAV occurs mainly through bites of arthropod vectors (tabanideo) which inoculate the virus into the animal's blood stream (mechanical transmission) when feeding themselves. The way of transmission is responsible for the high prevalence of EIA in areas favorable to the life cycle of vectors (ISSEL et al. Vet., 17:251-286, 1988). The EIAV can also be transmitted by the placenta and colostro of mares with high virus levels, and by needles and surgical instruments contaminated with blood (COGGINS Comparative diagnosis of viral diseases, NY, 4:646-658, 1981). The course of infection show different clinical forms of the disease (subacute, chronic and mainly inaparent or assimptomatic) in horses (ISSEL & COGGINS, J. Am. Vet. Med. Assoc. 174(7):727-33, 1979), and the most prominent signs are the feverish episodes, hemolytic anemia, anorexia, fast weight loss and ventral edema.
The laboratory diagnosis plays a decisive role in the control and the prevalence of assymptomatic carriers, non conclusive and possibility to confuse clinical diagnosis with other trypanosomiases, pyroplasmosis, leptospirosis, hepatitis and parasites.
The diagnosis of EIAV has been done though the detection of specific antibodies against surface antigens of virus present in the serum of affected animals using the Coggins or agar gel diffusion test (U.S. Pat. No. 3,929,982 and U.S. Pat. No. 3,932,601). In the Coggins test the antigen and serum sample are placed side by side in an agarose gel plate. If EIA antibodies are present in the test serum, they will form a precipitin line when diffusing toward the agarose gel
This methodology is inherently insensitive since EIAV antigen preparation derived from spleen of infected animals or equine derme cultures cells may be contaminated with non-EIAV antigens during its preparation. Besides, antibodies against non-EIAV antigens may be present in the test serum and can react with the non-EIA antigens forming a variety of nonspecific precipitin lines. Even if, the prepared EIAV-antigen batches can be purified the Coggins test is laborious, time-consuming and demanding of considerable expertise in interpretation of results. The Coggins test procedure takes twenty-four to forty-eight hours for the formation of clearly visible precipitin lines delaying results.
Porter, U.S. Pat. No. 4,806,467, discloses a method for detecting the EIA virus using a competitive enzyme-linked immunoabsorbent assay incorporating a purified viral antigen and a monoclonal antibody. To obtain the antigen, the EIA virus must first be cultured. The antigen used was the p26 capsid protein of the EIAV and was obtained through (purification of the cultured virus by a variety of means) well known in the art. The technique of virus tissue cultures increases the possibility of assay yield false positive results since the virus may be contaminated with other forms of protein or oven another virus. Additionally, the EIAV is hard to culture, making the Porter's approach difficult for large scale production.
The use of a synthetic peptide in an enzyme linked Immunosorbent assay for the detection of human immunodeficiency virus (HIV) was disclosed in Shoeman, R. L. et al, Analytical Biochemistry 161:370-379 (1987).
Darrel & Peisheng, tue U.S. Pat. No. 5,427,907, discloses a method to use a synthetic peptide as the antigen in an immunoassay for the detection of antibodies against the equine inectious anemia virus in the serum of horses. This procedure includes only the search of some epitopes of a virus proteins.
It is an object of the present invention to provide an assay for the detection of the equine infectious anemia virus antibodies which may be fast, easily and quickly performed by using the stable recombinant envelope protein (rgp26) which may be produced in sufficient amounts at a low cost.